Jasonsbrodies

Hello all! I have a 85 Z28 305 TPI Auto and I am having an backfiring issue that I can't seem to figure out the cause of it. I believe the backfiring is coming from under the hood but its hard to tell the sound is very loud and it almost sound like a burble tune or something. Being honest I don't know too much when it comes to working on cars but I try my best. Down below I will explain what happens when I drove it from the beginning when I start it up:
1. On start up there is rough Idling for about 20-30 seconds then it stops
2. Then I let it warm up and start driving
3. It drives fine for a little while (like a down the street trip) when I'm in between 30-40mph and when its stop and go traffic
4. Then when I'm going 45-60mph then that's when it starts to backfire
5. The backfiring happens only when I let off the gas pedal, I can also feel the car bogging, sputtering, hesitating to get up and go when I'm lightly pressing the gas pedal, but once again the backfiring only happens when I let off the gas pedal
6. Then I pull over and come to a complete stop and wait for maybe 5-10 seconds the proceed to drive, no backfire
7. Then everything from #3 down starts happening again
I've changed the spark plugs and wires because I did have a faulty wire (I changed all but cylinder 5 because I couldn't get to it), took it to the shop and they were saying the carbon build up was the cause of the backfire so they cleaned out the build up, but the issue still happens. If anyone has any idea on what could be causing the problem please I would greatly appreciate the help. Ask me any question I'll get to them as quick as I can I'll try to answer them and the best I can.

Thanks in advance!

Jasonsbrodies

Hello all! I have a 85 Z28 305 TPI Auto and I am having an backfiring issue that I can't seem to figure out the cause of it. I believe the backfiring is coming from under the hood but its hard to tell the sound is very loud and it almost sound like a burble tune or something. Being honest I don't know too much when it comes to working on cars but I try my best. Down below I will explain what happens when I drove it from the beginning when I start it up:
1. On start up there is rough Idling for about 20-30 seconds then it stops
2. Then I let it warm up and start driving
3. It drives fine for a little while (like a down the street trip) when I'm in between 30-40mph and when its stop and go traffic
4. Then when I'm going 45-60mph then that's when it starts to backfire
5. The backfiring happens only when I let off the gas pedal, I can also feel the car bogging, sputtering, hesitating to get up and go when I'm lightly pressing the gas pedal, but once again the backfiring only happens when I let off the gas pedal
6. Then I pull over and come to a complete stop and wait for maybe 5-10 seconds the proceed to drive, no backfire
7. Then everything from #3 down starts happening again
I've changed the spark plugs and wires because I did have a faulty wire (I changed all but cylinder 5 because I couldn't get to it), took it to the shop and they were saying the carbon build up was the cause of the backfire so they cleaned out the build up, but the issue still happens. If anyone has any idea on what could be causing the problem please I would greatly appreciate the help. Ask me any question I'll get to them as quick as I can I'll try to answer them and the best I can.

Thanks in advance!

T.L.

The first thing I would suspect is the AIR pump diverter valve.
Remove the belt for the AIR pump and see if the backfiring stops...

AlkyIROC

If you never changed the timing, moved the distributor etc and never replaced #5 plug wire, you may have a bad plug/wire in that position.

If you had replaced all the plug wires, I would suggest checking to see if 5 and 7 were mixed up but you said 5 was not replaced.

If you have a timing light, put the pickup onto every plug wire to make sure all the plugs are firing.

Or as mentioned above, a bad air pump diverter valve or even a bad EGR valve.

Jasonsbrodies

Thank you for the reply! The timing was set when I had the distributor changed sometime between September and October of last year 2023, and as for the plugs and wires, the bad plug/wire was on the even cylinder side (or left when facing the front of the engine), but I still completely understand what you are saying when it comes down to it. Unfortunately I do not have a timing light to check it so for now I will have to start with removing the belt from the air pump and checking the EGR valve. I will get back to you guys with an update.

Thank you again!

Jasonsbrodies

Thank you! I'll try that and get back to you with an update.

Aaron R.

Dude, you already have the exact same thread going in the Tech/General Engine section. You have even replied to people trying to help you in that one...

Edit: this reply was originally in reply to the same thread in a different tech section. The two threads are now combined with all replies together.

Jasonsbrodies

I apologize. I did originally posted this here but it did not show that it posted on my end. I apologize for the double posting.

Jasonsbrodies

Hey, I haven't gotten to the car yet, but I'm not sure on how to remove the belt exactly. I tried looking it up but couldn't find removals for my exact engine.
This is how mines is set up with 3 bolts on the air pump pulley. How would I go about giving it some slack to remove it?

daferris

iTrader: (8)

A bad ignition control module also can cause this.

EDGE

Jasonsbrodies

Ok, I was able to remove the air pump belt and when I took it on a few passes it drove fine but then it started to idle and drive rough, the engine was basically shaking the car, and started backfiring again. I also notice the temp gauge nearing 250F where usually its way under that. I'm thinking I'll check the EGR valve next and then the ICM.

T.L.

Well, that eliminates the AIR system. The ignition module just may be the culprit...